FIG. 1 is a block diagram of a power receiving unit of a conventional wireless power transmission system. Generally, a power receiving unit (PRU) has a structure as shown in FIG. 1. As well known, a resonator 10 is composed of an inductor L and a capacitor C, that is, an LC circuit, to receive wireless energy. In this case, an alternating current (AC) having the same frequency as that transmitted by a power transmission unit (PTU) flows through the resonator 10. Typically, a PRU converts received wireless energy into a stable DC signal as a final output to supply power to a load. Therefore, there is a need for a rectifier 20 as shown in FIG. 1. The rectifier 20 converts an AC signal into an unregulated DC signal. This signal is converted into a precise DC voltage Vout by means of a converter 30 and then supplied to a load. For reference, the converter 30 may be a buck-type converter, a boost-type converter, or a linear-type converter. Irrespective of the form of a converter, the converter has a two-stage structure as shown in FIG. 1. However, efficiency of a wireless power receiving unit is determined by multiplying efficiency of the rectifier 20 and efficiency of the converter 30. Accordingly, it may be difficult to satisfy high efficiency as the converter is configured to have multiple stages.